Electrocardiographic analysis of LV septum and lateral wall activation indicates LV systolic dysfunction and symptoms beyond apical rocking in left bundle branch block patients

Abstract The impact of left bundle branch block (LBBB) on cardiac function and prognosis varies with its anatomical location. A more proximal block leads to slow transseptal cell-to-cell activation, shifting the LV septal breakthrough site toward the apex, followed by a “U-turn” activation pattern and delayed lateral wall activation. On ECG, this may appear as a smaller R wave in aVF (reflecting apical breakthrough) and a larger R wave in aVL (reflecting delayed lateral wall activation). The difference between the intrinsicoid deflections (ID; time from QRS onset to R-wave peak) in these leads may represent the temporal delay between early and late LV activation. This electrical dyssynchrony produces uncoordinated LV deformation, visualized as apical rocking (ApR), which is associated with adverse remodeling and worse prognosis. Purpose We aimed to identify ECG parameters associated with LV systolic dysfunction and heart failure (HF) symptoms in patients with LBBB and to evaluate their relationship with ApR. Methods We analyzed 151 patients with LBBB in sinus rhythm, without significant coronary artery disease or more than mild valvular heart disease. ECG parameters included ID in leads aVL and aVF, their difference (Diff_aVL–aVFID), LV activation times (LVAT), and RR′ intervals in leads V4–V6, DI, and aVL. ApR was visually assessed in the apical 4 C-view as septal-to-lateral apical motion. HF symptoms were classified according to NYHA class and dichotomized into mild (I–II, 41%) and severe (III–IV, 59%). Patients were also categorized according to the presence (72%) or absence (28%) of ApR. Results Among ECG parameters, Diff_aVL_aVFID showed the strongest correlation with LV ejection fraction (LVEF) (r=0.717, p<0.001). In multivariable linear regression analysis, Diff_aVL_aVFID (p<0.001), severe NYHAclass (p<0.001), and indexed left atrial volume (LAVi) (p<0.001) were independently associated with LVEF (Figure 1). Diff_aVL_aVFID demonstrated good discriminatory performance for identifying patients with ApR (AUC=0.770). A cutoff value of 80 ms yielded 80% sensitivity and 70% specificity. For identifying patients with severe NYHAclass, the AUC was 0.858; the same 80 ms threshold provided 70% sensitivity and 86% specificity. Patients with severe NYHA class had significantly longer Diff_aVL_aVFID and shorter aVF ID (p<0.05). In multivariable logistic regression analysis, Diff_aVL–aVF_ID (p<0.001), LVEF (p<0.001), and LAVi (p=0.004) were independently associated with severe NYHA class. Conclusions Leads aVF and aVL provide insight into LV septal breakthrough and lateral wall activation patterns in patients with LBBB. The difference between ID in these leads (Diff_aVL–aVFID) was independently associated with LVEF and HF symptoms. A threshold value >80 ms was significantly associated with severe NYHAclass and the presence of ApR, suggesting its potential value as a marker of electrical–mechanical dyssynchrony in LBBB.